The present invention generally relates to a liquid crystal beam splitter and, in particular, relates to one such beam splitter that includes means for magnetically controlling the orientation of the liquid crystal molecules thereof.
The splitting of a light beam into the polarized components thereof by use of a layer of liquid crystal is known to be quite efficient. One particular design of such a light beam splitter has been described and discussed in U.S. patent application Ser. No. 795,150 filed on Nov. 5, 1985. This application is assigned to the assignee hereof and incorporated herein by reference. Therein a liquid crystal beam splitting cell is described that generally incorporates a number of index of refraction matching layers. These layers enhance the polarization splitting by reducing internal reflections and other deleterious effects caused by refraction index mismatches and impurities in the liquid crystal material. The directing of a light beam component according to the polarization thereof by a layer of liquid crystal material is, effectively, a bidirectional phenomena. Hence, by reversing the direction of the beam components, a liquid crystal beam splitter can be used as a beam combiner.
Liquid crystal beam splitters and combiners have been shown to be quite useful as a first and/or last stage of liquid crystal switching devices. Typical of such switching devices are those described in U.S. patent applications Ser. Nos. 795,157 and 795,154, both filed on Nov. 5, 1985 and assigned to the assignee hereof. These applications are incorporated herein by reference. The actual switching of a light beam, as discussed therein is effected by reorienting the polarization of the polarized components of a split incident light beam as the components pass through a polarization reorientation cell.
Since the liquid crystal beam splitting phenomena are primarily surface effects, the thickness of the liquid crystal material used in such devices is relatively small. The thickness of the liquid crystal material can, nevertheless, cause less than ideal beam splitting of an incident light beam. For example, for a layer of homeotropically aligned nematic liquid crystal material, the T.sub.E polarization of a light beam incident thereon at an angle at least equal to the critical angle is substantially completely reflected for a layer of liquid crystal material having a thickness greater than about 5 micrometers. However, such a thickness also effects the transmission of a small portion of T.sub.M polarized light along the same path as the T.sub.E polarized component. That is, the incident light beam is not perfectly split into the plane polarized components thereof. This impurity occurs because, in effect, the liquid crystal layer behaves similar to a well known Fabry-Perot etalon. Thus, the intensity of the unwanted reflected T.sub.M polarized component is a function of the reflection coefficients and the optical path difference (i.e. the thickness of the liquid crystal layer) between the reflections.
Consequently, in order to improve the beam splitting phenomena a liquid crystal beam splitter that reduces the reflected unwanted polarization component is highly desirable.